1. Field of the Invention
The present invention relates to an injection molding metal mold and an injection molding method using a thermoplastic synthetic resin. The invention particularly relates to an injection molding metal mold and an injection molding method by which a defective projection can be prevented from being produced in a gate-opposition surface of a molded product.
2. Related Art
Conventionally, molded products manufactured by injection molding have been widely used in various fields because the productivity thereof is generally high. Recently, the accuracy of injection molding has become so high that injection molded products have begun to be used in fields where injection molded products were not previously used.
One example of this is a plastic shutter for use in a magnetic disk cartridge. In such a shutter, a projection projecting near a base portion of the shutter can be guided in guide grooves formed along side edge portions of outer surfaces of upper and lower half-shells so as to open and close an opening for inserting a magnetic head. Conventionally, in almost all cases, a metal shutter formed of a metal plate such as thin stainless steel plate, or the like, and bent into a U-shape was used as such a shutter. Recently, however, a plastic shutter has often been used instead of a metal shutter because it can be manufactured easily and inexpensively.
A valve gate for mechanically closing a gate is effective in a thin molded product such as the afore-mentioned shutter. That is, in addition to shortening the molding cycle, high-speed filling-up of resin, fast gate sealing, and so on, there is not only an extremely infrequent occurrence of gate defects in appearance such as gate stringiness, gate residue or the like, but also there is an extremely small possibility of gate jam caused by contaminants in injected resin.
By using such a valve gate, it is possible to enlarge the gate diameter, so as to improve the flow of resin in the gate portion, for example, as disclosed in Japanese Patent Unexamined Publication No. Sho-2-229019. However, a gate having a large diameter allows a large size of and more amount of contaminants contained in a melted resin to pass through the gate easily, so that the contaminants are also injected into an injection cavity at a high speed. These contaminants collide against a portion of a movable mold opposite to the gate and shave to damage the surface of the metal mold. If the damage is great, it is transferred to the molded product, resulting in a fault in the appearance of the molded product. If this shaving is repeated, the portion of the metal mold portion opposite to the gate is gradually made concave, so that the corresponding portion of the molded product becomes convex. This is an extremely serious problem in the case where an inside sliding surface of the shutter becomes convex.
Not limited to solely the valve gate, the phenomenon wherein the portion opposite to the gate is damaged is apt to occur when a melted resin passes through the gate at a large rate. For a conspicuous example, it is apt to occur when a valve gate or an external heating hot runner using a hot spruce-gate or the other gate is used. In this case, the life of a metal mold is reduced by the formation of a concave portion on a surface of a cavity opposite to a gate. In addition, there has been a problem in that exchanging such a metal mold is expensive.
On the other hand, the following problem has been included in the molding of such a shutter.
This problem will be described with reference to FIGS. 9 and 10. FIGS. 9 and 10 are a perspective and a sectional side view illustrating the state where a metal mold for molding a shutter is opened and the shutter is ejected by ejection pins.
Movable mold 40 having a center core 41 as shown in FIG. 9 is provided with slide cores (not shown) which are provided so as to sandwich the center core 41 therebetween from right and left. For example, the side cores are opened suitably in accordance with the opening movement of the movable mold 40, and a molded shutter 22 is ejected by upper and lower ejection pins 42 (in the arrow A direction). In this state, the shutter 22 loses upper and lower supports (in fact, portions of the slide cores are opposite at a suitable distance), and the shutter 22 is supported only by the center core 41, so that the shutter 22 is inclined as shown by the arrow D. A degassing portion (not shown) is formed in a corner portion where the slide cores meet each other, and stains due to gas deposited to this portion. Therefore, at least a part of the stains may transfer and adhere to the shutter 22 because of the inclination of the shutter as mentioned above, or in the worst case, the shutter 22 may fall off. Since this ejected shutter 22 is usually caught by a shutter holding means (not shown), and moved to the next process step, such inclination or falling-off of the shutter becomes a serious problem.
The structure of the gate or the structure of degassing is disclosed, for example, in U.S. Pat. No. 4,990,077. The structure of a metal mold shown therein is a structure where a pin for degassing is provided at a place opposite to a gate, but it has no relation to and cannot solve the problem of damage to a gate-opposition surface and the problem when a molded product is ejected to be taken out and transferred to the next process step.